If you're uploading an image, there's a third option, Edit Image, which opens the image editor in the Assets dialog.

Click Upload.

The upload dialog shows the number of files remaining to be uploaded and the current speed of the upload in megabits per second. There is a progress bar for each file, along with the amount of time remaining for the upload. At any time before completion, you can click the "X" to the right of the progress bar to stop the selected files from uploading.

It's also possible, and usually more common, to upload assets at the same time that you add them to a page. Because adding assets to a page occurs through the Asset Manager, the steps are identical.